a) Field of the Invention
The present invention relates to a motor which has a rotor magnet having an index signal generating portion which generates an index signal to be detected by a magnetic resistance sensor while rotating together with a rotor.
b) Description of the Related Art
Among various compact motors such as capstan motors used for VCRs, audio tape recorders, etc., there is a widely known speed detection mechanism in which a index signal generating portion is formed on the outer peripheral surface of the rotor and a magnetic resistance sensor is arranged opposite in the vicinity to detect the speed of the rotor.
The magnetic resistance sensor is positioned at a position where the intensity of the magnetic field formed by the index signal generating portion becomes slightly lower than the rated sensitivity of the magnetic resistance sensor to obtain highly sensitive detection capability. For this reason, if the gap between the index signal generating portion and the magnetic resistance sensor deviates even a little, accordingly the amplitude of the output changes easily. Therefore, it is critical to minimize errors in gap as much as possible.
In a typical example, the gap between an index signal generating medium and a magnetic resistance sensor is set to be about 100 xcexcm. In this case, it is critical for the gap therebetween to be set within about xc2x130 xcexcm of tolerance to obtain a precise output from the magnetic resistance sensor. However, such a highly accurate positioning is very difficult to achieve.
The magnetic resistance sensor of the kind disclosed in, for example, Japanese Patent Kokai H1-203922 (1989), is mounted on a movable holder to determine an optimal position for the magnetic resistance sensor.
In other words, in the motor disclosed in the specification as shown in FIG. 10, an index signal generating medium 26, on which a plurality of magnets with alternate magnetic poles (S, N) are placed, is attached to the outer peripheral surface of a rotor 27, and the magnetic resistance sensor 21 is mounted on a holder 22 such that its magneto-sensitive surface faces the index signal generating medium 26. The holder 22 is constructed with a fixed holder which is secured on a printed circuit board 25 and a movable holder which is supported by the fixed holder in the radial direction of the motor.
While moving the magnetic resistance sensor 21 supported by the movable holder in the radial direction of the motor, the output signal from the magnetic resistance sensor 21 is monitored; based on the results, the magnetic resistance sensor 21 determines its optimal position. After determining the optimal position of the magnetic resistance sensor 21, a lead frame 24 of the movable holder on which the magnetic resistance holder 21 is loaded is secured on the printed circuit board 25 to fix the position of the magnetic resistance sensor 21.
In the motor of conventional as described above, it is critical to determine a highly accurate position of the magnetic resistance sensor with respect to the index signal generating portion. For this, a creative positioning is required. There are also many steps needed for determining the position. Further, the above mentioned method uses a holder mechanism in which a movable holder and a fixed holder are required for positioning the magnetic resistance sensor; this increases the number of components, because of the components needed for the mechanism, and the manufacturing cost accordingly.
Taking into account the above problems, the primary objective of the present invention is to provide a motor which can precisely detect the rotation speed and the like without requiring a highly accurate positioning of the magnetic resistance sensor of conventional technology.
In accordance with the invention, a motor comprises a rotor having an index signal generating portion on a rotor magnet and a magnetic resistance sensor having a magneto-sensitive surface. The sensor is positioned on a stator of the motor facing the index signal generating portion of the rotor with a predetermined gap. The sensor functions to sense an index signal in the magnetic field generated by the index signal generating portion during the rotation of the rotor. A speed detection circuit is included for detecting the speed of the motor based on the detection results of the magnetic resistance sensor. The index generating portion forms a magnetic area in which a magnetic flux intensity in a plane orthogonal to the principal magnetic flux of the index signal generating portion is greater than the rated sensitivity of the magnetic resistance sensor. The magnetic resistance sensor is positioned on the magnetized area such that its magneto-sensitive surface is parallel to the plane. The magnetic resistance sensor has an output which is in a saturated state.